Graphites are made up of layer planes of hexagonal arrays of networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size; the crystallites being highly aligned or oriented with respect to each other and possess well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that such graphites possess anisotropic structures and thus exhibit or possess many properties which are highly directional. Briefly, natural graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak Van der Waals forces.
The conventional process for producing flexible graphite sheet material e.g. web, paper, strip, tape, foil, mat or the like is described in U.S. Pat. No. 3,404,061 and briefly comprises treating the graphite particles with a suitable oxidizing agent to form soggy graphite particles which are heated to permit a natural expansion and then compressed or compacted together, in the absence of any binder, so as to form a flexible integrated graphite sheet of desired thickness and density. The compression or compaction is carried out by passing a thick bed of expanded particles between pressure rolls or a system of multiple pressure rolls to compress the material in several stages into sheet material of desired thickness. The compression operation flattens the expanded graphite particles causing them to engage and interlock. Obviously, any thickness can be realized by applying sufficient compressive force. However, the greater the compressive force the higher the density of the product. The density of the product determines the physical characteristics of the product such as deformability and stiffness. For applications where deformability is mandatory such as gasket seals the product standard density should be no greater than 1.3 g/cc and preferably in the range of 0.7 to 1.2 g/cc. Higher density product is too stiff for use as flexible sheet graphite where low density is necessary so that the product readily deforms and mashes into the irregularities of the surfaces to be sealed. Moreover, if the compressive force applied to the particles is excessive the particles split and separate causing weak spots which puncture to form pinholes when forming very thin sheets. Accordingly, for flexible graphite sheet application, the sheet thickness was heretofore substantially limited to a thickness in excess of about ten (10) mils. One recent proposal for forming thin sheet graphite material with a thickness equal to ten (10) mils is described in Japanese patent publication application No. 61(1986)-138865 entitled Method For Producing Flex Graphite Sheet. According to the Japanese publication a thin graphite sheet can be formed by applying an adhesive layer and expanded graphite on a polyester film, metal foil or paper tape before the graphite sheet is rolled down to the desired thickness of ten mils.
It has been discovered in accordance with the present invention that a sheet of ultra-thin flexible low density graphite of below eight mils in thickness can be formed without pinholes from natural graphite particles by increasing the degree of expansion of the particles during the exfoliation operation to produce particles which, prior to compression, have been expanded to a specific volume of at least 800 cc/gm. When the exfoliated graphite particles are expanded before compression to at least this minimum specific volume the density of the compressed sheet can be kept below 1.3 g/cc without increasing the susceptibility for forming pinholes during compression of the particles. The particles can then be compressed into ultra-thin sheet material of below 8 mils in thickness and preferably between 2-5 mils in thickness with a high degree of area/weight uniformity at a density of less than 1.3 g/cc, preferably between 0.7 to 1.2 g/cc. The process of the present invention is applicable primarily to natural graphite particles.